18 THE FROG 
of the alimentary canal (cesophagus, stomach, small and large 
intestine), showing in addition the liver, gall bladder (‘bile sac), 
pancreas and the course of the bile duct. The liver should be 
turned forward to expose the pancreas, etc. Examine a prepara- 
tion of the digestive system with the organs arranged in the 
proper position for sketching. 
B. The Finer Structure of the Alimentary Canal. 
1. Cross-section of the small intestine. (a) Under low 
power observe the shape of the cavity, and the five main layers 
of the wall, beginning from within: 
(1) The mucosa, or alimentary epithelium. Is there more 
than one layer of cells? Notice the closely packed nuclei. 
Notice the goblet cells scattered among the others; they may be 
recognized by the clear oval mass in their outer ends. 
(2) The submucosa, a layer consisting mainly of connective 
tissue (characterized by occasional small rounded cell-bodies and 
a large amount of loose fibrous intercellular substance) follows 
the folds of the mucosa. Within the submucosa are occasionally 
found large irregular lymph spaces. 
(3) The circular layer of non-striated muscle. This consists 
of slender spindle-shaped fibers each with an elongated nucleus. 
(4) The longitudinal layer of non-striated muscle. The 
fibers have the same structure as those of the circular layer, but 
are cut transversely. 
(5) The peritoneum or serous layer consists of a single layer 
of flattened cells cut transversely so that each cell appears spindle- 
shaped in the section. This layer forms the outer covering of 
the alimentary canal. 
(b) To show the finer structure draw only a small part of the 
section under high power. Select for this purpose a portion of 
the wall of the intestine including a small fold, and showing the 
above characteristic features as clearly as possible. 
2. Cross-section of the stomach. Study a transverse section 
taken through the cardiac region of the stomach. Under low 
power, identify and study the following layers, which are named 
in their order from within outward. 
(a) The mucosa. This is the innermost layer of the stomach 
and is so folded as to form long tubular glands set very closely 
